398.|398. Random Pick Index 398.RandomPickIndex

Given an array of integers with possible duplicates, randomly output the index of a given target number. You can assume that the given target number must exist in the array.
Note:
The array size can be very large. Solution that uses too much extra space will not pass the judge.

Example:int[] nums = new int[] {1,2,3,3,3}; Solution solution = new Solution(nums); // pick(3) should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning. solution.pick(3); // pick(1) should return 0. Since in the array only nums[0] is equal to 1. solution.pick(1);

Solution：Reservoir Sampling 思路：
Time Complexity: O(N) Space Complexity: O(N)
【398.|398. Random Pick Index】Solution Code：

public class Solution {int[] nums; Random rnd; public Solution(int[] nums) { this.nums = nums; this.rnd = new Random(); }public int pick(int target) { int result = -1; int count = 0; for (int i = 0; i < nums.length; i++) { if (nums[i] != target) continue; if (rnd.nextInt(++count) == 0) result = i; }return result; } }